The present invention is directed to communications-channel testing and in particular to detection of frame slips in frame-based channels.
An important figure of merit for a communications channel is the rate at which symbol errors occur in the signals that it conducts. Since the overwhelming majority of digital channels encode the information in choices between two voltage, frequency, or phase values, this figure of merit is known as the bit-error rate, which phrase we will accordingly use (not entirely precisely) in referring to the equivalent figure of merit for both binary- and non-binary-symbol channels.
In principle, bit-error rates are measured by simply applying a known symbol sequence as the channel input and then counting the number of symbols in the resultant channel output that do not match (a suitably delayed version of) the known transmitted sequence. The ratio of the number of incorrect symbols to the total number of received symbols is the bit-error rate.
Although measurement of bit-error rates is simple in principle, a straightforward application of the method just described can yield misleading results for some of the more-sophisticated, frame-based channels. Such channels use framing to keep track of what various parts of a received signal are intended to mean. In a multiplexed channel, for instance, one part of the signal may be data for one destination, another part may be data for another destination, and yet another part may be error-correction, diagnostic, or other housekeeping information to be used by the channel equipment itself.
To insure that information intended for one destination is not forwarded to a different destination or interpreted as housekeeping information, the channel equipment divides the channel signal into frames. Corresponding parts of all frames have the same purposes, and the channel equipment inserts into the frames unique patterns that establish the frame boundaries. If, in treating a given slice of the signal as a frame, channel equipment receiving the signal does not observe the known frame-establishing pattern in the predetermined locations, it "knows" that the slice is not really a frame and that its proposed frame boundaries should be slid along the signal until they define a slice in which the frame-indicating pattern is properly positioned.
Channel transmitters or repeaters must therefore transmit information in complete frames. However, transmitters and receivers in different channel links often employ separate clocks. The clocks are usually quite stable and accurate, but their frequencies nonetheless differ slightly in most systems. Data accordingly sometimes "pile up" or "run out" in a receiver's input buffer, and the receiver handles such situations by either slipping or repeating a frame in the signals that it forwards. This "slippage" is acceptable in normal operation of, say, voice channels, but it presents a problem in bit-error-rate measurement because a test system can report a large number of apparent errors if the timing of the expected pattern with which the test equipment compares the channel output is not adjusted for the frame "slip." The result is that the bit-error rate is greatly overstated.
Among the approaches that have been proposed for dealing with this problem is to count as only a single bit error bursts of errors that occur within a short interval. Such bursts are characteristic of frame slips; bit errors that result from frame slips ordinarily occur at a much greater rate than ordinary noise-based errors. However, frame slips are not the only causes of burst errors, so this approach provides only a rough compensation for the slip-error problem.
A more-direct approach, one that is based on explicit recognition of the frame size, is one in which successive segments of a predetermined test sequence are inserted at a given location in each successive transmitted frame. The segment of each frame into which the test segment has been inserted is monitored, and, since the inserted test sequence is known, a slip can be detected if a test-sequence segment is repeated or skipped. Such an approach is fairly effective at detecting slips, but it does not inherently provide a ready way to compensate for apparent bit errors that are counted before the slip is detected.
Another approach takes advantage of the properties of the shift-register-type pseudo-random-number generator that it employs to produce test input for the channel to be tested. Such pseudo-random-number generators apply as feedback to the first stage of a shift register the comparison of two selected subsequent stages, and the output of the last shift-register stage is the pseudo-random number. A generator at the receiving end identical to that at the transmitting end generates the expected sequence, and the test system ordinarily concludes that an error has occurred if a bit in the received signal differs from corresponding bits in the output of the receiver-side pseudo-random-number generator.
Without more, a slip could cause such a system to produce error indications. But that system's receiver has another shift register, whose first-stage input is the receiver signal. If the local pseudo-random-number generator does not successfully predict the received sequence but the comparison of the other shift register's selected stages does, the system recognizes that a slip has occurred. It thereby loads the second-shift-register contents into the local pseudo-random-number generator, and the expected-sequence signal is thereby resynchronized with the test-sequence signal. Such an approach limits the number of erroneous errors recorded, but it does not eliminate them.